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Anastrozole Reduces Postmenopausal Breast Cancer Risk Over Long Term

Twelve-year follow-up of the IBIS II study further supports the use of anastrozole for breast cancer prevention in high-risk postmenopausal women
18 Dec 2019
Aetiology, Epidemiology, Screening and Prevention;  Basic Scientific Principles;  Breast Cancer

Author: By Shreeya Nanda, Senior medwireNews Reporter

 

medwireNews: Long-term follow-up of the placebo-controlled IBIS II trial shows that anastrozole continues to be associated with a significantly reduced risk of breast cancer even in the post-treatment period. 

“Our results provide additional support for the use of anastrozole as the treatment of choice for breast cancer risk reduction in most postmenopausal women at high risk of developing breast cancer”, say the researchers who reported the 12-year data simultaneously in The Lancet and at the San Antonio Breast Cancer Symposium in Texas, USA. 

Over this period, the cumulative incidence of breast cancer was 5.3% among the 1920 participants of the double-blind trial who were randomly assigned to receive anastrozole 1 mg/day for 5 years and was 8.8% among their 1944 counterparts who were instead given placebo. 

This equated to a significant 49% reduction in the risk of developing breast cancer with the aromatase inhibitor. 

Jack Cuzick, from Queen Mary University London in the UK, and collaborators note that the magnitude of risk reduction associated with anastrozole was higher during the first 5 years, at 61% versus a 36% reduction in subsequent years. 

But they point out that the reduction in the later period remained statistically significant, and was “still greater than that observed for tamoxifen, which has produced a roughly constant 29% reduction for 20 years.” 

Moreover, the number needed to treat with anastrozole to prevent one case of breast cancer over the 12-year follow-up period was 29, “which compares favourably with the 58 needed for tamoxifen” at the same timepoint, notes the team. 

Anastrozole treatment was associated with a significant reduction in the risk of both invasive disease and ductal carcinoma in situ over the median 131 months of follow-up, with reductions relative to placebo of 47% and 59%, respectively. 

The decrease in invasive breast cancer risk was driven by the effect of anastrozole on oestrogen receptor (ER)-positive disease, with a significant 54% risk reduction versus placebo. However, Jack Cuzick and colleagues also observed an “unexpected”, albeit nonsignificant, 27% reduction in the risk for ER-negative disease, which they say “will need further follow-up to validate.” 

Additional follow-up will also be required to assess the effect of anastrozole on overall and breast cancer-specific mortality, which were both similar across the treatment arms at the time of the current analysis, say the researchers. 

There were no new major adverse events (AEs) over the longer follow-up, the team reports, noting that “a small non-significant” rise in the number of fractures with anastrozole in the first 5 years was balanced by a “slight reduction” in the subsequent follow-up period. 

The incidence of other major AEs, such as myocardial infarction and deep vein thrombosis, was comparable between groups. 

 

Reference 

Cuzick J, Sestak I, Forbes JF, et al. Use of anastrozole for breast cancer prevention (IBIS-II): long-term results of a randomised controlled trial. Lancet; Advance online publication 12 December 2019. doi: 10.1016/S0140-6736(19)32955-1

medwireNews (www.medwireNews.com) is an independent medical news service provided by Springer Healthcare. ©2019 Springer Healthcare part of the Springer Nature group

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